1. Field of the Invention
The present invention relates to a paper coating agent, and more particularly, it relates to a paper coating agent that imparts to paper water resistance, printability, surface strength, and barrier properties that make paper less permeable to air, oil, and organic solvents.
2. Description of the Prior Art
Heretofore, polyvinyl alcohol (abbreviated as PVA hereinafter) has been widely used as a clear coating agent to improve the surface strength, smoothness, gloss, and barrier properties of paper and as a binder for pigment coating compounds. It is also known that PVA is far superior to any other sizing agents in the film forming property and strength.
Recently, the surface strength of paper has tended to decrease as Asian tropical wood has come to account for a greater percentage in pulpwood and the printing speed has tended to increase. Under these conditions, there has arisen a demand for PVA that improves the surface strength and other properties of paper. In addition, in view of the remarkable development of barrier paper, there is a demand for PVA that imparts to paper improved barrier properties. Conventional PVA, however, does not meet completely these requirements. In order to overcome the disadvantage of conventional PVA, there was proposed in Japanese Patent Publication No. 39,442/1977 and Japanese Patent Laid-Open No. 20,698/1981 the use of PVA with a lactone ring or a carboxyl group introduced into the molecule. The proposed PVA is not effective in practical use. Thus the development of new PVA that meets the above-mentioned requirements has been expected.
New PVA is also required for making improved offset printing plates. Offset printing plates commonly used for office printing are made up of a substrate of wet-strength paper, an image forming layer, and an intermediate layer placed between them. The intermediate layer is provided to improve the durability of the plate. The image forming layer and intermediate layer should have a sufficient affinity for water to repel ink during printing as well as a sufficient resistance to damping water which is supplied repeatedly during printing. In order to provide both affinity for water and resistance to water, the conventional image forming layer and intermediate layer have been incorporated with a water-soluble polymeric substance such as PVA, casein, and starch, or a crosslink reaction product of a water-soluble polymer and an amino resin condensate. The crosslinking reaction, however, has a disadvantage. If the crosslinking reaction is allowed to take place before the compound is applied to the substrate, the compound becomes excessively viscous due to gellation, making coating impossible. On the other hand, the crosslinking reaction during or after the coating process requires such a high temperature and long time that the productivity is reduced to a great extent.
The above-mentioned system composed of water-soluble resin and crosslinking agent loses the balance of water affinity and water resistance when the mixing ratio of the two components varies or the crosslinking conditions vary. Thus, the offset printing plates prepared from it tend to vary in uniformity.